As one of a back frame structure of a vehicle seat, JP-A-2015-101285 discloses a structure in which an upper frame is connected in a bridging manner between upper end portions of side frames when forming a frame-shaped back frame serving as a framework of a seat back. The back frame is used for a three-point seat belt built-in type vehicle seat. In the back frame structure disclosed in JP-A-2015-101285, the upper frame is formed in a closed section with a substantially rectangular cross-section in order to counteract the load applied from the seat belt, and a holder for holding a headrest support is arranged in a through-hole drilled in an upper and lower direction. That is, the entire of the upper frame is formed in a closed section, thereby securing torsional rigidity.
In the above-described structure, the upper frame is a member that is formed by extrusion molding or the like of a light alloy and is entirely formed in a closed section. Accordingly, there is a problem that some of surface portions cannot be omitted even if they are not necessary for securing a predetermined torsional rigidity, and thus, it is difficult to achieve weight reduction. Therefore, it is conceivable that weight-reducing holes are drilled in the upper frame that is entirely formed in the closed section. However, this method is liable to lead to an increase in cost due to an increase in processing steps.